Hitherto, there has been proposed a parking device of this type, including a locking mechanism including a hydraulic piston 832 and a cylindrical tapered member 858 disposed on the same axis, the hydraulic piston 832 being coupled to a parking rod that operates in conjunction with a parking lock pawl that is engageable with a parking ratchet, and the tapered member 858 being fitted with a plunger of a solenoid 856 as illustrated in FIG. 16 and FIG. 17 which is a cross-sectional view taken along the line X-X of FIG. 16 (see Patent Document 1, for example). FIG. 16 illustrates the locking mechanism in the parking unlocked state. In the parking device, when the tapered member 858 is urged toward the hydraulic piston 832 (leftward in FIG. 16) by a spring 859 in the parking unlocked state, the hydraulic piston 832 is moved leftward in FIG. 16 with lug portions 862 formed on three plates 860 provided around the tapered member 858 pushed outward by the tapered member 858, which establishes the parking locked state. When the hydraulic piston 832 is moved rightward in FIG. 16 by a hydraulic pressure in the parking locked state and the tapered member 858 is pressed against the urging force of the spring 859, meanwhile, the lug portions 862 are closed inward to engage with a catching portion 834 of the hydraulic piston 832, and further the tapered member 858 is fixed in position by the solenoid 856 to lock the hydraulic piston 832 in position, which establishes the parking unlocked state. With such a configuration, the parking unlocked state can be held even when a hydraulic pressure for a hydraulic piston device is reduced with an engine-driven mechanical pump stopped during idle stop.